Introduce `Layer` trait

11 files changed, 224 insertions, 53 deletions

diff --git a/native/src/element.rs b/native/src/element.rs
index c6d65550..01379d2d 100644
--- a/native/src/element.rs
+++ b/native/src/element.rs
@@ -274,8 +274,9 @@ where
 
     pub fn overlay(
         &mut self,
-    ) -> Option<Box<dyn Overlay<Message, Renderer> + 'a>> {
-        self.widget.overlay()
+        layout: Layout<'_>,
+    ) -> Option<Overlay<'a, Message, Renderer>> {
+        self.widget.overlay(layout)
     }
 }
 
diff --git a/native/src/layer.rs b/native/src/layer.rs
new file mode 100644
index 00000000..d89fb4e5
--- /dev/null
+++ b/native/src/layer.rs
@@ -0,0 +1,34 @@
+use crate::{layout, Clipboard, Event, Hasher, Layout, Point, Size};
+
+pub trait Layer<Message, Renderer>
+where
+    Renderer: crate::Renderer,
+{
+    fn layout(
+        &self,
+        renderer: &Renderer,
+        bounds: Size,
+        position: Point,
+    ) -> layout::Node;
+
+    fn draw(
+        &self,
+        renderer: &mut Renderer,
+        defaults: &Renderer::Defaults,
+        layout: Layout<'_>,
+        cursor_position: Point,
+    ) -> Renderer::Output;
+
+    fn hash_layout(&self, state: &mut Hasher, position: Point);
+
+    fn on_event(
+        &mut self,
+        _event: Event,
+        _layout: Layout<'_>,
+        _cursor_position: Point,
+        _messages: &mut Vec<Message>,
+        _renderer: &Renderer,
+        _clipboard: Option<&dyn Clipboard>,
+    ) {
+    }
+}
diff --git a/native/src/layout.rs b/native/src/layout.rs
index 4a3ab94a..178f0f24 100644
--- a/native/src/layout.rs
+++ b/native/src/layout.rs
@@ -34,6 +34,13 @@ impl<'a> Layout<'a> {
         }
     }
 
+    /// Gets the position of the [`Layout`].
+    ///
+    /// [`Layout`]: struct.Layout.html
+    pub fn position(&self) -> Point {
+        self.position
+    }
+
     /// Gets the bounds of the [`Layout`].
     ///
     /// The returned [`Rectangle`] describes the position and size of a
diff --git a/native/src/lib.rs b/native/src/lib.rs
index dbec068d..6974c2bd 100644
--- a/native/src/lib.rs
+++ b/native/src/lib.rs
@@ -48,6 +48,7 @@ mod clipboard;
 mod element;
 mod event;
 mod hasher;
+mod layer;
 mod overlay;
 mod runtime;
 mod user_interface;
@@ -75,6 +76,7 @@ pub use debug::Debug;
 pub use element::Element;
 pub use event::Event;
 pub use hasher::Hasher;
+pub use layer::Layer;
 pub use layout::Layout;
 pub use overlay::Overlay;
 pub use program::Program;
diff --git a/native/src/overlay.rs b/native/src/overlay.rs
index d34432a4..a4bd5ea3 100644
--- a/native/src/overlay.rs
+++ b/native/src/overlay.rs
@@ -1,26 +1,41 @@
-use crate::{layout, Clipboard, Event, Hasher, Layout, Point};
+use crate::{layout, Clipboard, Event, Hasher, Layer, Layout, Point, Size};
 
-pub trait Overlay<Message, Renderer>
+#[allow(missing_debug_implementations)]
+pub struct Overlay<'a, Message, Renderer> {
+    position: Point,
+    layer: Box<dyn Layer<Message, Renderer> + 'a>,
+}
+
+impl<'a, Message, Renderer> Overlay<'a, Message, Renderer>
 where
     Renderer: crate::Renderer,
 {
-    fn layout(
-        &self,
-        renderer: &Renderer,
-        limits: &layout::Limits,
-    ) -> layout::Node;
+    pub fn new(
+        position: Point,
+        layer: Box<dyn Layer<Message, Renderer> + 'a>,
+    ) -> Self {
+        Self { position, layer }
+    }
 
-    fn draw(
+    pub fn layout(&self, renderer: &Renderer, bounds: Size) -> layout::Node {
+        self.layer.layout(renderer, bounds, self.position)
+    }
+
+    pub fn draw(
         &self,
         renderer: &mut Renderer,
         defaults: &Renderer::Defaults,
         layout: Layout<'_>,
         cursor_position: Point,
-    ) -> Renderer::Output;
+    ) -> Renderer::Output {
+        self.layer.draw(renderer, defaults, layout, cursor_position)
+    }
 
-    fn hash_layout(&self, state: &mut Hasher);
+    pub fn hash_layout(&self, state: &mut Hasher) {
+        self.layer.hash_layout(state, self.position);
+    }
 
-    fn on_event(
+    pub fn on_event(
         &mut self,
         _event: Event,
         _layout: Layout<'_>,
diff --git a/native/src/renderer.rs b/native/src/renderer.rs
index a16df72b..29a091a4 100644
--- a/native/src/renderer.rs
+++ b/native/src/renderer.rs
@@ -25,7 +25,7 @@ mod null;
 #[cfg(debug_assertions)]
 pub use null::Null;
 
-use crate::{layout, Element};
+use crate::{layout, Element, Rectangle};
 
 /// A component that can take the state of a user interface and produce an
 /// output for its users.
@@ -56,4 +56,11 @@ pub trait Renderer: Sized {
     ) -> layout::Node {
         element.layout(self, limits)
     }
+
+    fn overlay(
+        &mut self,
+        base: Self::Output,
+        overlay: Self::Output,
+        overlay_bounds: Rectangle,
+    ) -> Self::Output;
 }
diff --git a/native/src/renderer/null.rs b/native/src/renderer/null.rs
index 5fd3627b..bd6aca29 100644
--- a/native/src/renderer/null.rs
+++ b/native/src/renderer/null.rs
@@ -22,6 +22,9 @@ impl Null {
 impl Renderer for Null {
     type Output = ();
     type Defaults = ();
+
+    fn overlay(&mut self, _base: (), _overlay: (), _overlay_bounds: Rectangle) {
+    }
 }
 
 impl column::Renderer for Null {
diff --git a/native/src/user_interface.rs b/native/src/user_interface.rs
index b9646043..6758bce3 100644
--- a/native/src/user_interface.rs
+++ b/native/src/user_interface.rs
@@ -1,4 +1,4 @@
-use crate::{layout, Clipboard, Element, Event, Layout, Point, Size};
+use crate::{layout, Clipboard, Element, Event, Layout, Overlay, Point, Size};
 
 use std::hash::Hasher;
 
@@ -19,12 +19,17 @@ use std::hash::Hasher;
 /// [`UserInterface`]: struct.UserInterface.html
 #[allow(missing_debug_implementations)]
 pub struct UserInterface<'a, Message, Renderer> {
-    hash: u64,
-    root: Element<'a, Message, Renderer>,
-    layout: layout::Node,
+    base: Layer<Element<'a, Message, Renderer>>,
+    overlay: Option<Layer<Overlay<'a, Message, Renderer>>>,
     bounds: Size,
 }
 
+struct Layer<T> {
+    root: T,
+    layout: layout::Node,
+    hash: u64,
+}
+
 impl<'a, Message, Renderer> UserInterface<'a, Message, Renderer>
 where
     Renderer: crate::Renderer,
@@ -92,27 +97,45 @@ where
         cache: Cache,
         renderer: &mut Renderer,
     ) -> Self {
-        let root = root.into();
+        let mut root = root.into();
 
-        let hash = {
-            let hasher = &mut crate::Hasher::default();
-            root.hash_layout(hasher);
+        let (base, overlay) = {
+            let hash = {
+                let hasher = &mut crate::Hasher::default();
+                root.hash_layout(hasher);
 
-            hasher.finish()
-        };
+                hasher.finish()
+            };
 
-        let layout_is_cached = hash == cache.hash && bounds == cache.bounds;
+            let layout_is_cached = hash == cache.hash && bounds == cache.bounds;
 
-        let layout = if layout_is_cached {
-            cache.layout
-        } else {
-            renderer.layout(&root, &layout::Limits::new(Size::ZERO, bounds))
+            let layout = if layout_is_cached {
+                cache.layout
+            } else {
+                renderer.layout(&root, &layout::Limits::new(Size::ZERO, bounds))
+            };
+
+            let overlay = root.overlay(Layout::new(&layout));
+
+            (Layer { root, layout, hash }, overlay)
         };
 
+        let overlay = overlay.map(|root| {
+            let hash = {
+                let hasher = &mut crate::Hasher::default();
+                root.hash_layout(hasher);
+
+                hasher.finish()
+            };
+
+            let layout = root.layout(&renderer, bounds);
+
+            Layer { root, layout, hash }
+        });
+
         UserInterface {
-            hash,
-            root,
-            layout,
+            base,
+            overlay,
             bounds,
         }
     }
@@ -193,14 +216,42 @@ where
         let mut messages = Vec::new();
 
         for event in events {
-            self.root.widget.on_event(
-                event,
-                Layout::new(&self.layout),
-                cursor_position,
-                &mut messages,
-                renderer,
-                clipboard,
-            );
+            if let Some(overlay) = &mut self.overlay {
+                overlay.root.on_event(
+                    event.clone(),
+                    Layout::new(&overlay.layout),
+                    cursor_position,
+                    &mut messages,
+                    renderer,
+                    clipboard,
+                );
+
+                let base_cursor =
+                    if overlay.layout.bounds().contains(cursor_position) {
+                        // TODO: Encode cursor availability
+                        Point::new(-1.0, -1.0)
+                    } else {
+                        cursor_position
+                    };
+
+                self.base.root.widget.on_event(
+                    event,
+                    Layout::new(&self.base.layout),
+                    base_cursor,
+                    &mut messages,
+                    renderer,
+                    clipboard,
+                );
+            } else {
+                self.base.root.widget.on_event(
+                    event,
+                    Layout::new(&self.base.layout),
+                    cursor_position,
+                    &mut messages,
+                    renderer,
+                    clipboard,
+                );
+            }
         }
 
         messages
@@ -280,12 +331,42 @@ where
         renderer: &mut Renderer,
         cursor_position: Point,
     ) -> Renderer::Output {
-        self.root.widget.draw(
-            renderer,
-            &Renderer::Defaults::default(),
-            Layout::new(&self.layout),
-            cursor_position,
-        )
+        if let Some(overlay) = &self.overlay {
+            let overlay_bounds = overlay.layout.bounds();
+
+            let base_cursor = if overlay_bounds.contains(cursor_position) {
+                Point::new(-1.0, -1.0)
+            } else {
+                cursor_position
+            };
+
+            let base_primitives = self.base.root.widget.draw(
+                renderer,
+                &Renderer::Defaults::default(),
+                Layout::new(&self.base.layout),
+                base_cursor,
+            );
+
+            let overlay_primitives = overlay.root.draw(
+                renderer,
+                &Renderer::Defaults::default(),
+                Layout::new(&overlay.layout),
+                cursor_position,
+            );
+
+            renderer.overlay(
+                base_primitives,
+                overlay_primitives,
+                overlay_bounds,
+            )
+        } else {
+            self.base.root.widget.draw(
+                renderer,
+                &Renderer::Defaults::default(),
+                Layout::new(&self.base.layout),
+                cursor_position,
+            )
+        }
     }
 
     /// Extract the [`Cache`] of the [`UserInterface`], consuming it in the
@@ -295,8 +376,8 @@ where
     /// [`UserInterface`]: struct.UserInterface.html
     pub fn into_cache(self) -> Cache {
         Cache {
-            hash: self.hash,
-            layout: self.layout,
+            hash: self.base.hash,
+            layout: self.base.layout,
             bounds: self.bounds,
         }
     }
diff --git a/native/src/widget.rs b/native/src/widget.rs
index dd1a97d2..0494636f 100644
--- a/native/src/widget.rs
+++ b/native/src/widget.rs
@@ -176,7 +176,10 @@ where
     ) {
     }
 
-    fn overlay(&mut self) -> Option<Box<dyn Overlay<Message, Renderer> + 'a>> {
+    fn overlay(
+        &mut self,
+        _layout: Layout<'_>,
+    ) -> Option<Overlay<'a, Message, Renderer>> {
         None
     }
 }
diff --git a/native/src/widget/column.rs b/native/src/widget/column.rs
index 6afc94d6..9a6dbdb3 100644
--- a/native/src/widget/column.rs
+++ b/native/src/widget/column.rs
@@ -2,8 +2,8 @@
 use std::hash::Hash;
 
 use crate::{
-    layout, Align, Clipboard, Element, Event, Hasher, Layout, Length, Point,
-    Widget,
+    layout, Align, Clipboard, Element, Event, Hasher, Layout, Length, Overlay,
+    Point, Widget,
 };
 
 use std::u32;
@@ -204,6 +204,17 @@ where
             child.widget.hash_layout(state);
         }
     }
+
+    fn overlay(
+        &mut self,
+        layout: Layout<'_>,
+    ) -> Option<Overlay<'a, Message, Renderer>> {
+        self.children
+            .iter_mut()
+            .zip(layout.children())
+            .filter_map(|(child, layout)| child.widget.overlay(layout))
+            .next()
+    }
 }
 
 /// The renderer of a [`Column`].
diff --git a/native/src/widget/container.rs b/native/src/widget/container.rs
index 3ad12eeb..3bdb1a27 100644
--- a/native/src/widget/container.rs
+++ b/native/src/widget/container.rs
@@ -2,8 +2,8 @@
 use std::hash::Hash;
 
 use crate::{
-    layout, Align, Clipboard, Element, Event, Hasher, Layout, Length, Point,
-    Rectangle, Widget,
+    layout, Align, Clipboard, Element, Event, Hasher, Layout, Length, Overlay,
+    Point, Rectangle, Widget,
 };
 
 use std::u32;
@@ -214,6 +214,13 @@ where
 
         self.content.hash_layout(state);
     }
+
+    fn overlay(
+        &mut self,
+        layout: Layout<'_>,
+    ) -> Option<Overlay<'a, Message, Renderer>> {
+        self.content.overlay(layout.children().next().unwrap())
+    }
 }
 
 /// The renderer of a [`Container`].